It is known from the state of the art, evident in the issued patents enumerated above, that the exhaust gases of a high-pressure blast furnace can be cleaned by passing the exhaust gases in succession through a coarse-particle separator, a prewashing column and a differential-pressure washer, the gas/water mixture being accelerated through an annular gap in the latter washer and expanded downstream thereof to effect an intimate contact between the gas and the water, thereby leading to high-efficiency washing of the gas. The annular-gap washer or differential-pressure washer generally comprises a tubular passage formed with a constriction in which a body is displaceable, thereby defining an annular gap traversed by the water-laden gas stream. The width of this gap can be adjusted by shifting the body axially within the passage, thereby varying the pressure differential across this washer.
In improvements over the state of the art developed by us and our coworkers, an expansion turbine unit is provided downstream of the differential-pressure washer to convert the kinetic energy and potential energy of the scrubbed gas into useful work, e.g. the generation of electrical energy.
It is possible in such a system to control the pressure at the gas outlet or head of a high-pressure blast furnace by an appropriate control circuit having a sensor responsive to this pressure and a controller with a positioning member which varies the flow cross-section along the path of the gas to maintain a substantially constant backpressure at the blast furnace.
We have found that it is possible to provide the differential-pressure washer as the exclusive control element for the control circuit which maintains the exhaust-gas pressure at the head of the high-pressure blast furnace constant. For example, the differential-pressure washer can comprise two differential-pressure washing units, one of which is provided directly upstream of the expansion turbine unit while the other is provided in series with the first. The differential-pressure washers are then provided with a bypass duct which branches from the main stream beyond the differential-pressure washer and is connected to a pure gas discharge duct. The bypass can be provided with a control valve and with the expansion turbine.
This arrangement has the advantage that for the highest pressures at the head of the furnace, the expansion turbine is effective for bypass control of the pressure and thus is driven to convert part of the gas energy to useful work. At the lower pressures the use of the bypass is reduced by the control valve.
The expansion turbine could, of course, have a quick-closing valve as is conventional in turbine installations and as described below.
It was found that the aforedescribed system, using an annular-gap washer, was highly advantageous since it allowed the exhaust-gas pressure at the head of the high-pressure blast furnace to be maintained without difficulty when the quick-closing valve of the expansion turbine was closed. The turbine unit could be used to maintain the gas pressure more or less constant at least within certain limits and an effective combination of maximum energy recovery and gas cleaning was obtained.
However, experience with our earlier system has demonstrated that it is not always completely satisfactory when control of both the backpressure at the head of the high-pressure blast furnace and optimum differential-pressure washing is desired. This can be understood in terms of the requirements for differential-pressure washing and backpressure control. For optimum differential-pressure washing, it is desirable to maintain the pressure differential across the annular gap of the washer substantially constant in spite of fluctuations in volume rate of flow of the gases and the instantaneous pressures ahead of or behind the gap.
For control of the gas pressure at the head of the highpressure blast furnace, however, it may be necessary to discharge (vent) sharply larger volumes of gas from the blast furnace to prevent an excessively high pressure from developing or to reduce the volume rate of flow of the gas below the optimum flow to achieve the aforementioned pressure differential at the differential-pressure washer. Accordingly, even with the earlier system, which represented a major advance in the art, we were not able to fully achieve all of the desiderata for optimum regulation of blast-furnace backpressure and optimum operation of the differential-pressure washer.